In non-invasive imaging systems, X-ray tubes are used in various X-ray systems and computed tomography (CT) systems as a source of X-ray radiation. The radiation is emitted in response to control signals during an examination or imaging sequences. Typically, the X-ray tube includes a cathode and an anode. An emitter within the cathode may emit a stream of electrons in response to heat resulting from an applied electrical current via the thermionic effect, and/or an electric field resulting from an applied voltage to a properly shaped metallic plate in front of the emitter. The anode may include a target that is impacted by the stream of electrons. The target may, as a result of impact by the electron beam, produce X-ray radiation and heat.
In such imaging systems, the radiation passes through a subject of interest, such as a patient, baggage, or an article of manufacture, and a portion of the radiation impacts a digital detector or a photographic plate where the image data is collected. In some X-ray systems the photographic plate is then developed to produce an image which may be used by a quality control technician, security personnel, a radiologist or attending physician for diagnostic purposes. In digital X-ray systems a photodetector produces signals representative of the amount or intensity of radiation impacting discrete elements of a detector surface. The signals may then be processed to generate an image that may be displayed for review. In CT systems a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is rotated about a patient. In certain configurations, a series of these signals may be used to generate a volumetric image. Generally, the quality of the volumetric image is dependent on the ability of the X-ray source and the X-ray detector to quickly generate data as they are rotated on the gantry.
In other systems, such as systems for oncological radiation treatment, a source of X-rays may be used to direct ionizing radiation toward a target tissue. In some radiation treatment configurations, the source may also include an X-ray tube. X-ray tubes used for radiation treatment purposes may also include a thermionic emitter and a target anode that generates X-rays, such as described above. Such X-ray tubes or sources may also include one or more collimation features for focusing or limiting emitted X-rays into a beam of a desired size or shape. The X-ray source may be displaced about (e.g., rotated about) the target tissue while maintaining the focus of the X-ray beam on the tissue of interest, which allows a substantially constant X-ray flux to be provided to the target tissue while minimizing X-ray exposure to outlying tissue.